<template>
  <ul class="list">
    <li class="item" v-for="item of letters" :key="item" :ref="item"
     @touchstart="handleTouchStart"
     @touchmove="handleTouchMove"
     @touchend="handleTouchEnd"
     @click="handleLetterClick">{{item}}</li>
  </ul>
</template>

<script>
export default {
  name:'CityAlphabet',
  props: {
    cities:Object
  },
  computed: {
    letters () {
      const letters = []
      for(let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
  },
  data () {
    return {
      touchStatus: false,
      startY: 0,
      timer: null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods: {
    handleLetterClick (e) {
      this.$emit('change',e.target.innerText)
    },
    handleTouchStart () {
      this.touchStatus = true
    },
    handleTouchMove (e) {
      if(this.touchStatus) {
          if(this.timer) {
            clearTimeout(this.timer)
          }
          this.timer = setTimeout(() => {
            const touchY = e.touches[0].clientY - 79
            const index = Math.floor((touchY - this.startY) / 20)
            if(index >= 0 && index < this.letters.length) {
              this.$emit('change',this.letters[index])
            }
          },16)
        }
    },
    handleTouchEnd () {
      this.touchStatus = false
    }
  }

}
</script>

<style lang="stylus" scoped>
.list
  display flex
  flex-direction column
  justify-content center
  position absolute
  right 0
  top 1.58rem
  bottom 0
  width .4rem
  .item
    text-align center
    line-height .4rem
    color #00bcd4
</style>
